Climate change threatens coral reef
fishes in myriad ways, but maybe not in all the ways we thought. Some studies have
suggested that ocean acidification, one consequence of climate change, might
warp fish behavior. But new research shows that fish may be far more resilient.

Scientists predict that as atmospheric
carbon dioxide levels continue to rise, and oceans absorb even more, the waters
will increasingly acidify. About a decade ago, a series of high-profile studies
alarmed biologists with reports of severe behavioral impairments in coral fishes
exposed to mildly acidified water. Larval fish lost the ability
to smell predators
and became dangerously hyperactive and confused when exposed to ocean
acidification levels projected for 2100 if fossil fuel use continues at current
levels (SN: 7/6/10). Research into the
effects of ocean acidification then ballooned, becoming one of
the most studied subjects in marine science

But a three-year attempt to
replicate and improve upon some of those earlier studies paints a starkly
different picture. Tests of over 900 individuals from six different species of
coral reef fish showed that exposure to acidified waters had no significant adverse
effects on fish activity or predator avoidance
researchers report January 8 in Nature.

“Climate change is a huge threat to
reef fishes,” says Timothy Clark, a comparative physiologist at Deakin
University in Geelong, Australia. But “the acidification levels we’ll see by the end
of this century aren’t going to have any real impact on fish, even beyond just
coral reef fish.”

Other biologists question such strong
statements. The study is the most robust yet to find no effect from
acidification, and the researchers’ “data is unimpeachable,” says Andrew
Esbaugh, a comparative physiologist at University of Texas at Austin who was not
involved in this study nor in those this study sought to replicate. But because
the study looked only at six reef fish species, “their conclusion that
acidification won’t affect fish behavior is a bit of an overreach,” he says.

Alarming studies

Early research linking behavioral impairments
to ocean acidification was dramatic. “These papers were, without a doubt, some
of the most incredible things I’ve ever seen in biology,” Clark says.

One 2010 paper found a near complete reversal of chemical cue
after exposure to slightly acidified water. After just days,
lab-reared larval clownfish became attracted to the scent of their predators,
preferring it 90 percent of the time.

clownfish anemone
Some of the foundational studies linking ocean acidification to behavioral impairments in fish used clownfish, pictured here near a sea anemone, along with other fish species. The current study that sought to replicate this earlier work did not use clownfish, since the authors say populations at their field site have declined in recent years. Researchers involved in the earlier studies say this difference makes this current study incomparable to their work. The current study authors argue that there was enough overlap in species they did use to be a fair comparison.Fototrav/getty images plus

Another study that year found similar
impairments in clownfish and damselfish. Up to 90 percent of exposed fish were gobbled up when
transplanted to a live reef, compared with 10 percent mortality for fish never
exposed to acidified water. 

“These are just mind-blowing results,” Clark
says. Studies since have reported more variability, including many seeing no effect
of ocean acidification. But Clark and his colleagues wanted to replicate the
earlier studies, he says, partly to understand the physiology behind these
aberrant behaviors.

Differing conclusions

From 2014 to 2016, Clark’s team collected over
900 individuals from six different species of coral reef fish in Townsville,
Australia, where much of the previous work was done. Most fish were wild-caught
adults or juveniles, while a subset came from a local aquarium.

After the first round of tests on five species
in 2014, the researchers did not see any effects. “At first I was quite
skeptical of our results,” says Josefin Sundin, a behavioral ecologist at the
Swedish University of Agricultural Studies in Stockholm. If the effects are
truly as severe as earlier studies found, she thought they should be easy to

Over the next two field seasons, the
researchers tested even more fish, and at different life stages. Each
experimental trial was recorded on video; automated tracking software was used to
measure behaviors in an effort to improve upon previous, more subjective,
methods of tracking, the researchers say.

All told, the study found little evidence that
elevated CO₂ impairs fish behavior. Spiny chromis
damselfish juveniles showed a slightly impaired ability to discriminate between
predator and non-predator chemical cues. But other fish exposed to acidified
waters avoided the scent of predators, like normal fish. The
researchers also found little evidence of hyperactivity or other behavioral

“After three years of experimentation, we’re
confident in our data,” Sundin says. These papers showing large effects from
acidification are still being cited as the foundation of this field, she says.
“If they’re incorrect, perhaps we should shift our focus to studying other climate
change–related stressors besides ocean acidification,” such as heat waves, she

‘Apples to oranges’ or ‘to apples’?

Danielle Dixson, a behavioral ecologist at the University
of Delaware in Newark, conducted some of the 2010 research while a Ph.D.
student in the lab of Philip Munday at James Cook University in Townsville. She
doesn’t question the new results, but says it’s not a fair replication. “It’s
comparing apples to oranges,” she says.

Much of her team’s early work included
clownfish, which the current study did not use. And these early studies used
larval fish raised in the lab, while this study used mostly wild-caught adults
or juveniles. “Those two groups have very different life experiences,” she says.
“My fish were naïve to different acidification levels or predator cues, while
wild-caught fish are not.” 

Dixson notes that the field of ocean acidification’s
effect on fish behavior has advanced significantly since these studies were
published. “We’ve improved a lot on those early studies, and subsequent
research suggests that the effects of ocean acidification may not be as drastic
as initially reported,” she says. “That doesn’t negate the earlier work.”

But Clark and Sundin contend that their work
replicated the crucial elements of earlier studies. They used four of the same
species and tested some of the same life stages as previous work. While some
earlier studies looked only at lab-reared fish, Sundin says the same research
group and others also reported behavioral impairments from ocean acidification
in wild-caught individuals in over a dozen published studies. “It’s certainly a
comparison of apples and apples,” Clark says.

Browman, a marine scientist at the Institute of Marine Research in Bergen,
Norway, who wasn’t involved in any of these studies, calls the new paper “a healthy
step forward in rebalancing the picture” of the effects of ocean acidification
on fish behavior.

The field already is shifting towards the view that ocean
acidification is one of many challenges fishes face from climate change, he
says, and in many cases it’s probably less important than factors like ocean
heat waves.